Protein-disulphide isomerase and prolyl isomerase act differently and independently as catalysts of protein folding

Cyclophilin A Inhibitor Debio-025 Targets Crk, Reduces Metastasis, and Induces Tumor Immunogenicity in Breast Cancer

The Crk adaptor protein, a critical modifier of multiple signaling pathways, is overexpressed in many cancers where it contributes to tumor progression and metastasis. Recently, we have shown that Crk interacts with the peptidyl prolyl cis-trans isomerase, Cyclophilin A (CypA; PP1A) via a G²¹⁹P²²⁰Y²²¹ (GPY) motif in the carboxyl-terminal linker region of Crk, thereby delaying pY221 phosphorylation and preventing downregulation of Crk signaling. Here, we investigate the physiologic significance of the CypA/Crk interaction and query whether CypA inhibition affects Crk signaling in vitro and in vivo. We show that CypA, when induced under conditions of hypoxia, regulates Crk pY221 phosphorylation and signaling in cancer cell lines. Using nuclear magnetic resonance spectroscopy, we show that CypA binds to the Crk GPY motif via the catalytic PPII domain of CypA, and small-molecule nonimmunosuppressive inhibitors of CypA (Debio-025) disrupt the CypA-CrkII interaction and restores phosphorylation of Crk Y221. In cultured cell lines, Debio-025 suppresses cell migration, and when administered in vivo in an orthotopic model of triple-negative breast cancer, Debio-025 showed antitumor efficacy either alone or in combination with anti-PD-1 mAb, reducing both tumor volume and metastatic lung dispersion. Furthermore, when analyzed by NanoString immune profiling, treatment of Debio-025 with anti-PD-1 mAb increased both T-cell signaling and innate immune signaling in tumor microenvironment. IMPLICATIONS: These data suggest that pharmacologic inhibition of CypA may provide a promising and unanticipated consequence in cancer biology, in part by targeting the CypA/CrkII axis that regulates cell migration, tumor metastasis, and host antitumor immune evasion.

Posttranslational modifications and proteinopathies: how guardians of the proteome are defeated

Protein folding is one of the fundamental processes in life and therefore needs to be tightly regulated. Many cellular quality control systems are in place to ensure that proteostasis is optimally adjusted for a changing environment, facilitating protein folding, translocation and degradation. These systems include the molecular chaperones and the major protein degradation systems, namely the ubiquitin proteasome system and autophagy. However, the capacity of the quality control systems can be exhausted and protein misfolding and aggregation, including the formation of amyloids, can occur as a result of ageing, mutations or exogenous influences. There are many known diseases in which protein misfolding and aggregation can be the underlying cause of the pathological condition; these are referred to as proteinopathies. Over the last decade, it has become clear that posttranslational modifications can govern and modulate protein folding, and that aberrant posttranslational modifications can cause or contribute to proteinopathies. This review provides an overview of protein folding and misfolding and the role of the major protein quality control systems. It focusses on different posttranslational modifications and gives examples of how these posttranslational modifications can alter protein folding and cause or accompany proteinopathies.

A fluorescent peptidyl substrate for visualizing peptidyl-prolyl cis/trans isomerase activity in live cells

This communication reports on a fluorescent probe (PPI-P) for imaging active peptidyl-prolyl cis/trans isomerases in live cells. PPI-P is capable of responding to both recombinant and cellular PPIases fluorogenically, and has been shown to specifically image active PPIases in live cells.

Hydrolytic inhibition of α-chymotrypsin by 2,8,14,20-tetrakis(D-leucyl-D-valinamido)resorc[4]arenecarboxylic acid: a spectroscopic NMR and computational combined approach

The stereochemical features of 2,8,14,20-tetrakis(D-leucyl-D-valinamido)resorc[4]arenecarboxylic acid and the N-succinyl-L-alanyl-L-alanyl-L-prolyl-L-phenylalanine-4-nitroanilide polypeptide substrate were investigated by nuclear magnetic resonance spectroscopy. Proton selective relaxation parameters gave the basis for the inhibitory activity of resorcin[4]arene in the hydrolysis of the polypeptide substrate by α-chymotrypsin. Results showed that an interaction between the resorcin[4]arene and α-chymotrypsin does occur, and involves the hydrophobic moiety of the macrocycle. This interaction is further reinforced by polar groups located on the side chains of the resorcin[4]arene, whereas the macrocycle-polypeptide substrate interaction is negligible. Conformational analysis and interaction studies carried out by molecular modeling are in good agreement with the NMR data, thus providing an additional support to the rationalization of the inhibitory potential of resorcin[4]arenes on the α-chymotrypsin activity.

The life of ribulose 1,5-bisphosphate carboxylase/oxygenase--posttranslational facts and mysteries

The life of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), from gene to protein to irreplaceable component of photosynthetic CO2 assimilation, has successfully served as a model for a number of essential cellular processes centered on protein chemistry and amino acid modifications. Once translated, the two subunits of Rubisco undergo a myriad of co- and posttranslational modifications accompanied by constant interactions with structurally modifying enzymes. Even after final assembly, the essential role played by Rubisco in photosynthetic CO2 assimilation is dependent on continuous conformation modifications by Rubisco activase. Rubisco is also continuously assaulted by various environmental factors, resulting in its turnover and degradation by processes that appear to be enhanced during plant senescence.

Folding mechanism of indole-3-glycerol phosphate synthase from Sulfolobus solfataricus: a test of the conservation of folding mechanisms hypothesis in (beta(alpha))(8) barrels

As a test of the hypothesis that folding mechanisms are better conserved than sequences in TIM barrels, the equilibrium and kinetic folding mechanisms of indole-3-glycerol phosphate synthase (sIGPS) from the thermoacidophilic archaebacterium Sulfolobus solfataricus were compared to the well-characterized models of the alpha subunit of tryptophan synthase (alphaTS) from Escherichia coli. A multifaceted approach combining urea denaturation and far-UV circular dichroism, tyrosine fluorescence total intensity, and tyrosine fluorescence anisotropy was employed. Despite a sequence identity of only 13%, a stable intermediate (I) in sIGPS was found to be similar to a stable intermediate in alphaTS in terms of its thermodynamic properties and secondary structure. Kinetic experiments revealed that the fastest detectable folding event for sIGPS involves a burst-phase (<5ms) reaction that leads directly to the stable intermediate. The slower of two subsequent phases reflects the formation/disruption of an off-pathway dimeric form of I. The faster phase reflects the conversion of I to the native state and is limited by folding under marginally stable conditions and by isomerization or rearrangement under strongly folding conditions. By contrast, alphaTS is thought to fold via an off-pathway burst-phase intermediate whose unfolding controls access to a set of four on-pathway intermediates that comprise the stable equilibrium intermediate. At least three proline isomerization reactions are known to limit their interconversions and lead to a parallel channel mechanism. The simple sequential mechanism deduced for sIGPS reflects the dominance of the on-pathway burst-phase intermediate and the absence of prolyl residues that partition the stable intermediate into kinetically distinguishable species. Comparison of the results for sIGPS and alphaTS demonstrates that the thermodynamic properties and the final steps of the folding reaction are better conserved than the early events. The initial events in folding appear to be more sensitive to the sequence differences between the two TIM barrel proteins.

Prolyl isomerases in a minimal cell. Catalysis of protein folding by trigger factor from Mycoplasma genitalium

Peptidyl-prolyl cis/trans isomerases (PPIases) catalyze the isomerization of prolyl peptide bonds. Distinct families of this class of enzymes are involved in protein folding in vitro, whereas their significance in free living organisms is not known. Previously, we inspected the smallest known genome of a self-replicating organism and found that Mycoplasma genitalium is devoid of all known PPIases except the trigger factor. Despite the extensive sequence information becoming available, most genes remain hypothetical and enzyme activities in many species have not been assigned to an open reading frame. Therefore, we studied the PPIase activity in crude extracts of M. genitalium. We showed that this is solely attributed to a single enzyme activity, the trigger factor. Characterization of this enzyme revealed that its PPIase activity resides in a central 12-kDa domain. Only the complete trigger factor is able to cis/trans isomerize extended peptide substrates, while the PPIase domain alone can not. The N- and the C-terminal domains of the trigger factor seem to function in binding of proteins as substrates, as demonstrated by protein refolding experiments, in which the complete trigger factor catalyzed protein refolding towards a model protein 500-fold more efficiently than the isolated central PPIase domain. Protein modeling studies suggest that the PPIase domain can fold in a similar way as the PPIase domain of FK506 binding proteins (FKBPs), one class of PPIases, despite only very limited sequence homology. Differences at the active site explain why this enzyme is not inhibited by FK506 in contrast with FKBPs. Trigger factor expressed in Escherichia coli confirms its additional chaperone functions, as shown by its association with chaperones GroEL and GroES after induction of misfolding. In contrast, the isolated PPIase-domain lacks any association with chaperones from E. coli. In summary, trigger factor of M. genitalium is the single folding isomerase of this organism, which harbors an enzymatically active PPIase domain with structural homology to FKBPs. Its additional domains confer its ability to be an efficient catalyst of protein folding. The protein folding machinery is conserved and shows a dual function as a chaperone and a prolyl isomerase.

Characterization of the cyclophilin of Trichophyton mentagrophytes

A genetic approach to cyclophilins in a dermatophyte, Trichophyton mentagrophytes, was carried out. The nucleotide and deduced amino acid sequences of the cyclophilin of T. mentagrophytes shared about 70% sequence similarity with those of Schizosaccharomyces pombe, Saccharomyces cerevisiae and Candida albicans. However, the first 21 amino acid and the C-terminal amino acid regions of 188 to 226 of the T. mentagrophytes cyclophilin were distinct from those of the other fungal cyclophilins. The recombinant glutathione S-transferase (GST)-T. mentagrophytes cyclophilin fusion protein produced by Escherichia coli was purified. The protease digest of the fusion protein had a molecular weight of about 13 kDa and peptidyl-prolyl cis-trans isomerase (PPI) activity. This digest protein from T. mentagrophytes was confirmed to be cyclophilin by proving PPI activity.

A two-dimensional protein map of Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells are used extensively for the expression of biopharmaceutical protein products. As part of our effort to better understand CHO cell physiology and protein expression changes caused by modified culture conditions, we have begun to map CHO cell polypeptides. A parental cell line reference map was established using two-dimensional gel electrophoresis with immobilized pH gradients (pH 3-10) in the first dimension and a linear acrylamide gradient (9-18%T) in the second dimension. The map is composed of over 1000 silver-stained protein spots. Protein identification is proceeding using a combination of immunostaining, NH2-terminal sequencing, and mass spectrometric analyses. Among the proteins so far identified are glucose-regulated protein 78 (GRP78), protein disulfide isomerase (PDI), galectin-1, and several heat-shock proteins. The goal is to generate a database which emphasizes those proteins most relevant to the use of CHO cells as a host for recombinant protein expression.

An enzymatically active truncated form (-55 N-terminal residues) of rabbit gastric lipase. Correlation between the enzymatic activity and disulfide bond oxydo-reduction state

Rabbit gastric lipase (RGL) was subjected to proteolysis with trypsin and led to cleavage occurring at three defined sites (Lys-4, Arg-55 and Arg-229). The tryptic hydrolysate contained four fragments: Gly-230-Lys-379 (T1), Gly-56-Arg-229 (T2), Ser-5-Arg-55 (T3), as well as a 45 kDa molecular form consisting of peptides T1 and T2 linked by a disulfide bridge. The tryptic hydrolysate of RGL as well as the 55 N-terminal amino acid deleted forms conserved 30% of the initial enzymatic activity in a tributyrin assay. Two out of the three cysteine residues which are present in all the known gastric lipases were found to be involved in a disulfide bridge. Unlike HGL, RGL appears to have a heterogenous pattern of cysteine residues. The 30% enzymatic activity of RGL persisting after trypsin treatment may be attributable to the 45 kDa molecular form (with the Cys-227-Cys-236 or Cys-227-Cys-244 disulfide bridge). Trypsin-treated HGL, which was completely inactivated, showed that a single location of the disulfide bridge existed between cysteine residues 236 and 244. It can be concluded that the existence of one disulfide bridge is necessary to maintain the lipase activity of the 45 kDa form of RGL.

Antibody catalysis of peptidyl-prolyl cis-trans isomerization in the folding of RNase T1

An antibody generated to an alpha-keto amide containing hapten 1 catalyzes the cis-trans isomerization of peptidyl-prolyl amide bonds in peptides and in the protein RNase T1. The antibody-catalyzed peptide isomerization reaction showed saturation kinetics for the cis-substrate, Suc-Ala-Ala-Pro-Phe-pNA, with a kcat/Km value of 883 s-1.M-1; the reaction was inhibited by the hapten analog 13 (Ki = 3. 0 +/- 0.4 microM). Refolding of denatured RNase T1 to its native conformation also was catalyzed by the antibody, with the antibody-catalyzed folding reaction inhibitable both by the hapten 1 and hapten analog 13. These results demonstrate that antibodies can catalyze conformational changes in protein structure, a transformation involved in many cellular processes.

Elevated expression of PDI family proteins during differentiation of mouse F9 teratocarcinoma cells

We investigated the expression of protein disulfide isomerase family proteins (PDI, ERp61, and ERp72) in mouse F9 teratocarcinoma cells during differentiation induced by treatment with retinoic acid and dibutyryl cAMP. Each member of this family was expressed at a constitutive level in undifferentiated F9 cells. During differentiation of F9 cells to parietal or visceral endodermal cells the protein level of all these enzymes increased, although the extent of this increase in both protein and mRNA levels varied among the enzymes. Certain proteins were found to be coimmunoprecipitated with PDI, ERp61, and ERp72 in the presence of a chemical crosslinker. Type IV collagen was significantly coprecipitated with PDI whereas laminin was equally coprecipitated with the three proteins. Furthermore, 210 kDa protein characteristically coprecipitated with ERp72. Thus, the induction of PDI family proteins during the differentiation of F9 cells and their association with different proteins may implicate specific functions of each member of this family despite the common redox activity capable of catalyzing the disulfide bond formation.

Cyclosporin A selectively reduces the functional expression of Kir2.1 potassium channels in Xenopus oocytes

The immunosuppressant cyclosporin A (CsA) reduced the functional expression of Kir2.1 potassium channels in Xenopus oocytes in a dose-dependent manner with an IC50 of 11 microM when the oocytes were incubated with CsA after RNA injection. FK506 was less effective than CsA; cyclosporin H, a non-immunosuppressive derivative of CsA, did not have a significant effect. CsA did not impair protein synthesis since other potassium channel types (Kir1.1, Kv1.1, Kv1.4) were much less sensitive to CsA. Our results suggest that the functional expression of Kir2.1 channels is facilitated by the peptidyl-prolyl isomerase cyclophilin. The observations illustrate a new role of CsA in regulation of membrane ion transport, and may provide an alternative explanation for CsA-induced side effects in clinical use.

Chemical synthesis and structural characterization of the RGD-protein decorsin: a potent inhibitor of platelet aggregation

Decorsin is a 39-residue RGD-protein crosslinked by three disulfide bridges isolated from the leech Macrobdella decora belonging to the family of GPIIb-IIIa antagonists and acting as a potent inhibitor of platelet aggregation. Here we report the solid-phase synthesis of decorsin using the Fmoc strategy. The crude polypeptide was purified by reverse-phase HPLC in its reduced form and allowed to refold in the presence of glutathione. The homogeneity of the synthetic oxidized decorsin was established by reverse-phase HPLC and capillary zone electrophoresis. The results of amino acid analysis after acid hydrolysis of the synthetic protein, NH2-terminal sequencing and mass determination (4,377 Da) by electrospray mass spectrometry were in full agreement with this theory. The correct pairing of the three disulfide bridges in synthetic decorsin was determined by a combined approach of both peptide mapping using proteolytic enzymes and analysis of the disulfide chirality by CD spectroscopy in the near-UV region. Synthetic decorsin inhibited human platelet aggregation with an IC50 of approximately 0.1 microM, a figure quite similar to that determined utilizing decorsin from natural source. In particular, the synthetic protein was 2,000-fold more potent than a model RGD-peptide (e.g., Arg-Gly-Asp-Ser) in inhibiting platelet aggregation. Thermal denaturation experiments of synthetic decorsin, monitored by CD spectroscopy, revealed its high thermal stability (Tm approximately 74 degrees C). The features of the oxidative refolding process of reduced decorsin, as well as the thermal stability of the oxidized species, were compared with those previously determined for the NH2-terminal core domain fragment 1-41 or 1-43 from hirudin. This fragment shows similarity in size, pairing of the three disulfides and three-dimensional structure with those of decorsin, even if very low sequence similarity. It is suggested that the less efficient oxidative folding and the enhanced thermal stability of decorsin in respect to those of hirudin core domain likely can be ascribed to the presence of the six Pro residues in the decorsin chain, whereas none is present in the hirudin domain. The results of this study indicate that decorsin can be obtained by solid-phase methodology in purity and quantities suitable for structural and functional studies and thus open the way to prepare by chemical methods novel decorsin derivatives containing unusual amino acids or even non-peptidic moieties.

Peptidyl prolyl cis-trans isomerase activity of cyclophilin A in functional homo-oligomeric receptor expression

The functional expression of homo-oligomeric alpha7 neuronal nicotinic and type 3 serotonin receptors is dependent on the activity of a cyclophilin. In this paper we demonstrate that the mechanism of cyclophilin action during functional homo-oligomeric receptor expression in Xenopus oocytes is distinct from the calcineurin-dependent immunosuppressive mechanism by showing that a nonimmunosuppressive analog of cyclosporin A (CsA), SDZ 211-811, reduces functional receptor expression to the same extent as CsA. The cytoplasmic subtype of cyclophilin, cyclophilin A (CyPA), appears to be required for functional receptor expression. This is because overexpression of CyPA and a CyPA mutant that is deficient in CsA binding activity reverses CsA-induced reduction in functional receptor expression. The mechanism of action of CyPA is likely to involve its prolyl isomerase activity because a mutant CyPA with a single amino acid substitution (arginine 55 to alanine) that is predicted to produce a 1000-fold attenuation in isomerase activity fails to reverse the cyclosporin A effect. Our data also suggest that CyPA does not form a stable complex with receptor subunits.

Catalysis of the refolding of urea denatured creatine kinase by peptidyl-prolyl cis-trans isomerase

The effect of peptidyl-prolyl cis-trans isomerase (PPIase) on the refolding and reactivation courses of urea-denatured creatine kinase was followed by fluorescence emission, ultraviolet difference spectra and recovery of activity. PPIase is shown to accelerate the slow-phasic reaction of the refolding of urea-denatured creatine kinase. The results suggest that the prolyl peptide bond isomerization may be one of the rate-determining steps in the refolding of creatine kinase.

Purification and characterization of cytosolic and microsomal cyclophilins from maize (Zea mays)

Methods for the purification and separation of peptidyl prolyl cis-trans isomerase (PPI) from cytosolic and microsomal fractions of etiolated maize are described. On SDS/PAGE, the purified preparations appears as single polypeptides with molecular masses of 17.5 kDa and 17.7 kDa respectively. Instead of using immobilized cyclosporin A derivatives as affinity adsorbents, our methods employ conventional techniques enabling purification of the proteins on a much larger scale than previously described. An antiserum raised against the cytosolic PPI recognizes polypeptides of similar molecular mass from a wide range of plant species on an immunoblot. There is virtually no recognition of the microsomal PPI. The cytosolic and microsomal PPIs are inhibited by cyclosporin A (Ki = 6 nM in both cases), indicating that they are cyclophilins. The cytosolic enzyme is inactivated by 5 mM N-ethylmaleimide and 2 mM phenylglyoxal. N-terminal sequencing of the microsomal PPI indicates a high level of sequence similarity with the N-terminal sequence of mature animal s-cyclophilin (cyclophilin B).

A kinetic analysis of the folding of human carbonic anhydrase II and its catalysis by cyclophilin

The kinetics of unfolding and refolding of human carbonic anhydrase II (HCAII) and its catalysis by the peptidyl-prolyl-cis/trans-isomerase cyclophilin were investigated. HCAII contains 15 trans- and 2 cis-prolyl peptide bonds, and, when long-term denatured, virtually all unfolded molecules contain non-native prolyl isomers. In unfolding these molecules (Us) are produced slowly in a biphasic process reflecting the isomerization of several trans-prolines and of one cis-proline. In refolding, the rapid formation of an intermediate of the molten globule type is followed by several slow prolyl isomerizations, which determine the rate of reactivation. By a short 10-s incubation in 5.0 M guanidinium chloride at 2 degrees C, unfolded HCAII species with all prolines still in the native conformation (Uf) could be produced. Surprisingly, only a fraction of Uf refolds rapidly, but the other molecules refold slowly. Evidently, some prolyl peptide bonds isomerize early in refolding, at the stage of the molten globule and as a consequence, molecules with incorrect prolyl isomers are formed in competition with the productive folding of Uf. This fraction of slow-folding molecules is strongly increased when cyclophilin is present, because it accelerates the formation of non-native prolyl isomers as long as the molecules remain in the molten globule state. Later cyclophilin catalyzes the isomerization of these prolyl peptide bonds toward the native state, which are stabilized in their conformation by further folding to the native state. This catalysis is very efficient, because only prolines that are accessible in the molten globule are involved in this sequence of isomerization and reisomerization.

A gene of Acinetobacter calcoaceticus BD413 encodes a periplasmic peptidyl-prolyl cis-trans isomerase of the cyclophilin sub-class that is not essential for growth

Downstream of the Acinetobacter calcoaceticus estA gene, encoding a cell-bound esterase, an open reading frame (orf) was identified, which may encode a protein with a mass of 20.4 kDa. This protein shows extensive similarity to both prokaryotic and eukaryotic peptidyl-prolyl cis-trans isomerases (PPIases) of the cyclophilin sub-class, especially to the periplasmic rotamase (RotA) of Escherichia coli. A putative signal sequence suggests that the product of the Acinetobacter gene, we termed rotA, is located outside the cytoplasm. Transcription of the gene is initiated from a promoter, just upstream of the rotA orf. The observation that two A. calcoaceticus rotA deletion mutants display no apparent mutant phenotype, suggests that this PPIase is not essential for growth of the organism. These mutants, to our knowledge, are the first prokaryotic PPIase mutants reported.

Prolyl isomerase requirement for the expression of functional homo-oligomeric ligand-gated ion channels

Ligand-gated ion channel subunits show a striking abundance of highly conserved proline residues. We, therefore, tested the hypothesis that peptidyl-prolyl isomerases may be involved in the maturation of these channels. Cyclosporin A, a selective blocker of a ubiquitous isomerase cyclophilin, reduced the surface expression in Xenopus oocytes of functional homo-oligomeric receptors containing nicotinic acetylcholine receptor subunit alpha 7 without blocking alpha 7 polypeptide synthesis. This effect could be generalized to the homo-oligomeric 5-hydroxytryptamine type 3 receptor but not to the hetero-oligomeric muscle nicotinic receptor. An alpha 7 receptor could be rescued from cyclosporin A blockade by coexpressed muscle non-alpha subunits. The effect of cyclosporin A was reversed by overexpression of exogenous rat brain cyclophilin. These findings indicate that cyclophilins may play a critical role in the maturation of homo-oligomeric receptors, acting directly or indirectly as prolyl isomerases or as molecular chaperones.

Prolyl isomerases catalyze antibody folding in vitro

Some slow-folding phases in the in vitro refolding of proteins originate from the isomerization of prolyl-peptide bonds, which can be accelerated by a class of enzymes called prolyl isomerases (PPIs). We used the in vitro folding of an antibody Fab fragment as a model system to study the effect of PPI on a folding reaction that is only partially reversible. We show here that members of both subclasses of PPIs, cyclophilin and FK 506 binding protein (FKBP), accelerate the refolding process and increase the yield of correctly folded molecules. An acceleration of folding was not observed in the presence of the specific inhibitor cyclosporin A, but still the yield of correctly folded molecules was increased. Bovine serum albumin (BSA) increased the yield comparable to cyclophilin but, in contrast, did not influence the rate of reactivation. These effects were observed only when cyclophilin or BSA were present during the first few seconds of refolding. However, the rate-limiting reactivation reaction is still accelerated when PPI is added several minutes after starting refolding. In contrast, the prokaryotic chaperone GroEL influences the refolding yield when added several minutes after initiating refolding. The results show that PPIs influence the folding of Fab in two different ways. (1) They act as true catalysts of protein folding by accelerating the rate-limiting isomerization of Xaa-Pro peptide bonds. Proline isomerization is obviously a late folding step and has no influence on the formation of aggregates within the first seconds of the refolding reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

The cis/trans interconversion of the calcium regulating hormone calcitonin is catalyzed by cyclophilin

The cytosolic peptidyl-prolyl cis/trans isomerase cyclophilin from pig kidney can accelerate catalytically the cis/trans isomerization of prolyl peptide bonds. One- and two-dimensional 1H NMR spectroscopy was used to prove that the polypeptide hormone calcitonin is a substrate for cyclophilin. Isomerization of only one of the two prolyl peptide bonds is catalyzed significantly. The efficiency of catalysis was calculated by lineshape analysis and NOESY spectroscopy. Cyclosporin A completely blocks the effect of the enzyme on the conformational dynamics of the polypeptide.

Protein-specific S-thiolation in human endothelial cells during oxidative stress

Confluent human umbilical vein endothelial cells were treated with diamide, t-butyl hydroperoxide (t-BH) or the hydrogen peroxide generating system glucose/glucose oxidase and the effects on glutathione oxidation and protein S-thiolation were examined. In the presence of all three oxidants glutathione was rapidly oxidized to a similar extent and S-thiolation of a limited number of proteins occurred. Diamide caused considerable S-thiolation of proteins with molecular masses of 44, 34, 24 and 14 kDa, of which the protein with molecular mass of 44 kDa was most extensively modified. t-BH caused extensive modification of proteins with molecular masses of 24 and 14 kDa whilst hydrogen peroxide caused S-thiolation of proteins of 39, 24 and 14 kDa. This study shows that S-thiolation of proteins is an important metabolic response to oxidant insult in human endothelial cells and that the specificity of the response depends on the chemical nature of the oxidant.

Peptidyl-prolyl cis-trans isomerase improves the efficiency of protein disulfide isomerase as a catalyst of protein folding

The cis-trans isomerization of prolyl peptide bonds and the formation of disulfide bonds are both slow steps in protein folding. By using ribonuclease T1 as a model system, we show that these two processes can become linked in the oxidative folding of reduced proteins and that the formation of the correct disulfide bonds is facilitated in the presence of peptidyl-prolyl cis-trans isomerase. In particular, the efficiency of protein disulfide isomerase (EC 5.3.4.1) as a catalyst of disulfide bond formation in the course of oxidative folding is markedly improved when peptidyl-prolyl cis-trans isomerase is present simultaneously. Possibly, unfolded or partially folded protein chains with correct prolyl isomers are better substrates for catalysis by protein disulfide isomerase. The interdependence of the two enzymatic activities detected during in vitro folding experiments could be of importance for the de novo folding and disulfide bond formation of nascent proteins in the endoplasmic reticulum.

Comparison between avian and human prolyl 4-hydroxylases: studies on the holomeric enzymes and their constituent subunits

Prolyl 4-hydroxylase, a key enzyme in collagen biosynthesis, catalyzes the conversion of selected prolyl residues to trans-hydroxyproline in nascent or completed pro-alpha chains of procollagen. The enzyme is a tetramer composed of two nonidentical subunits, designated alpha and beta. To compare the enzyme and its subunits from different sources, the chick embryo and human placental prolyl 4-hydroxylases were purified to homogeneity and their physicochemical and immunological properties were determined. Both enzymes were glycoproteins with estimated apparent molecular weights ranging between 400 and 600 kDa. Amino acid and carbohydrate analyses showed slight differences between the two holomeric enzymes, consistent with their deduced amino acid sequences from their respective cDNAs. Human placental prolyl 4-hydroxylase contained more tightly bound iron than the chick embryo enzyme. Immunodiffusion of the human placental enzyme with antibodies raised against the purified chick embryo prolyl 4-hydroxylase demonstrated partial identity, indicating different antigenic determinants in their tertiary structures. The enzymes could be separated by high-resolution capillary electrophoresis, indicating differential charge densities for the native chick embryo and human placental proteins. Electrophoretic studies revealed that the human prolyl 4-hydroxylase is a tetrameric enzyme containing two nonidentical subunits of about 64 and 62 kDa, in a ratio of approximately 1 to 2, designated alpha and beta, respectively. In contrast, the chick embryo alpha and beta subunit ratio was 1 to 1. Notably, the human alpha subunit was partially degraded when subjected to electrophoresis under denaturing conditions. Analogously, when the chick embryo enzyme was subjected to limited proteolysis, selective degradation of the alpha subunit was observed. Finally, only the alpha subunit was bound to Concanavalin A demonstrating that the alpha subunits of prolyl 4-hydroxylase in both species were glycosylated. Using biochemical techniques, these results demonstrated that the 4-trans-hydroxy-L-proline residues in human placental collagens are synthesized by an enzyme whose primary structure and immunological properties differ from those of the previously well-characterized chick embryo enzyme, consistent with their recently deduced primary structures from cDNA sequences.

Murine cyclophilin-S1: a variant peptidyl-prolyl isomerase with a putative signal sequence expressed in differentiating F9 cells

Fractionation of differentiating murine teratocarcinoma F9 cells and extraction of the nuclear/microsomal pellets with ethidium bromide led to the purification and microsequencing of the protein mCyP-S1, a novel cyclosporin A-sensitive peptidyl-prolyl cis-trans isomerase (PPIase). mCyP-S1 is a new member of the cyclophilin class of proteins. Cloning and sequencing of the mCyP-S1 cDNA revealed extended coding capacity for a putative N-terminal signal sequence, suggesting processing of mCyP-S1 during intracellular translocation across the membrane of the endoplasmic reticulum. mCyP-S1 is abundantly expressed in a variety of mouse organ tissues and its mRNA levels increase during F9 cell differentiation. Specific subcellular localization of PPIases is postulated to contribute to functional specificities of this class of enzymes.

Effects of cyclosporine A on chick osteoclasts in vitro

We have studied the effects of cyclosporine A (CsA) on basal and bovine parathyroid hormone (1-34) (bPTH)-stimulated bone resorption by osteoclasts in 24-hour cultures of chick long bone cells. At a high concentration (10 micrograms/ml), CsA had a cytotoxic effect on both osteoclasts and mononuclear cells in the culture. At 1 microgram/ml, CsA inhibited basal and bPTH-stimulated bone resorption but was not cytotoxic over 24 hours. We also studied the binding of bPTH to the osteoblastic cell line, Saos-2, and chick long bone cells in suspension culture. CsA inhibited bPTH binding in Saos-2 in a dose-dependent manner; inhibition of binding was also observed in chick bone cells. The effects of CsA on osteoclast viability and resorptive function may be due to a direct effect on the osteoclasts and/or to an interaction with the nonosteoclastic cell population in the culture.

Nuclear association of a T-cell transcription factor blocked by FK-506 and cyclosporin A

Cyclosporin A and FK506 inhibit T- and B-cell activation and other processes essential to an effective immune response. In T lymphocytes these drugs disrupt an unknown step in the transmission of signals from the T-cell antigen receptor to cytokine genes that coordinate the immune response. The putative intracellular receptors for FK506 and cyclosporin are cis-trans prolyl isomerases. Binding of the drug inhibits isomerase activity, but studies with other prolyl isomerase inhibitors and analysis of cyclosporin-resistant mutants in yeast suggest that the effects of the drug result from the formation of an inhibitory complex between the drug and isomerase, and not from inhibition of isomerase activity. A transcription factor, NF-AT, which is essential for early T-cell gene activation, seems to be a specific target of cyclosporin A and FK506 action because transcription directed by this protein is blocked in T cells treated with these drugs, with little or no effect on other transcription factors such as AP-1 and NF-kappa B. Here we demonstrate that NF-AT is formed when a signal from the antigen receptor induces a pre-existing cytoplasmic subunit to translocate to the nucleus and combine with a newly synthesized nuclear subunit of NF-AT. FK506 and cyclosporin A block translocation of the cytoplasmic component without affecting synthesis of the nuclear subunit.

An endoplasmic reticulum-specific cyclophilin

Cyclophilin is a ubiquitously expressed cytosolic peptidyl-prolyl cis-trans isomerase that is inhibited by the immunosuppressive drug cyclosporin A. A degenerate oligonucleotide based on a conserved cyclophilin sequence was used to isolate cDNA clones representing a ubiquitously expressed mRNA from mice and humans. This mRNA encodes a novel 20-kDa protein, CPH2, that shares 64% sequence identity with cyclophilin. Bacterially expressed CPH2 binds cyclosporin A and is a cyclosporin A-inhibitable peptidyl-prolyl cis-trans isomerase. Cell fractionation of rat liver followed by Western blot (immunoblot) analysis indicated that CPH2 is not cytosolic but rather is located exclusively in the endoplasmic reticulum. These results suggest that cyclosporin A mediates its effect on cells through more than one cyclophilin and that cyclosporin A-induced misfolding of T-cell membrane proteins normally mediated by CPH2 plays a role in immunosuppression.

An endoplasmic reticulum-specific cyclophilin

Cyclophilin is a ubiquitously expressed cytosolic peptidyl-prolyl cis-trans isomerase that is inhibited by the immunosuppressive drug cyclosporin A. A degenerate oligonucleotide based on a conserved cyclophilin sequence was used to isolate cDNA clones representing a ubiquitously expressed mRNA from mice and humans. This mRNA encodes a novel 20-kDa protein, CPH2, that shares 64% sequence identity with cyclophilin. Bacterially expressed CPH2 binds cyclosporin A and is a cyclosporin A-inhibitable peptidyl-prolyl cis-trans isomerase. Cell fractionation of rat liver followed by Western blot (immunoblot) analysis indicated that CPH2 is not cytosolic but rather is located exclusively in the endoplasmic reticulum. These results suggest that cyclosporin A mediates its effect on cells through more than one cyclophilin and that cyclosporin A-induced misfolding of T-cell membrane proteins normally mediated by CPH2 plays a role in immunosuppression.

Determination of kinetic constants for peptidyl prolyl cis-trans isomerases by an improved spectrophotometric assay

The kinetic properties and substrate specificity of two well-characterized peptidyl prolyl cis-trans isomerases (PPIases), cyclophilin and the FK-506 binding protein (FKBP), have been previously examined [Fischer, G., Bang, H., Berger, E., & Schellenberger, A. (1984) Biochim. Biophys. Acta 791, 87-97; Harrison, R.K., & Stein, R.L. (1990) Biochemistry 29, 1684-1689; Albers, M.W., Walsh, C.T., & Schreiber, S. L. (1990) J. Org. Chem. 55, 4984-4986]. The chymotrypsin-coupled enzymatic assay employed in these studies suffers from two serious shortcomings. Due to the low equilibrium population of the X-cis-Pro-Phe-pNA isomer (the PPIase substrate), in conjunction with the low solubility of p-nitroaniline generated by chymotrypsin hydrolysis, substrate concentrations in the saturating region are not experimentally attainable. Secondly, the uncatalyzed cis-trans isomerization obscures the interpretation of the initial velocity. As a result of these limitations, the steady-state kinetic parameters (Km,Kcat) have not been determined. Here we introduce an improved version of the spectrophotometric assay and report for the first time the Michaelis constants and turnover numbers for both PPIases with established substrates. The improvements in the experimental conditions originate in a medium-induced increase in the equilibrium population of the cis X-Pro conformer and in conducting the assay at 0 degrees C to suppress the uncatalyzed thermal isomerization. In addition, we present a rigorous mathematical model of the spectrophotometric progress curves that accounts for the contributions of the residual background rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptidyl-prolyl cis-trans isomerase activity as studied by dynamic proton NMR spectroscopy

Recently the identity of the peptidyl-prolyl cis-trans isomerase (PPIase), which accelerates the cis/trans isomerization of prolyl peptide bonds and cyclophilin, the binding protein for the immunosuppressive drug Cyclosporin A (CsA), was discovered. The PPIase catalysis toward the substrate Suc-Ala-Phe-Pro-Phe-pNA has been studied by 1H NMR spectroscopy. Using the bandshape analysis technique the rate of interconversion between the cis and trans isomers of the substrate could be measured in the presence of PPIase and under equilibrium conditions. The acceleration is inhibited by equimolar amounts of CsA. The results provide evidence that the PPIase catalysis is more complex than a simple exchange between two states.

Comparison of the activities of protein disulphide-isomerase and thioredoxin in catalysing disulphide isomerization in a protein substrate

1. The activities of protein disulphide-isomerase (PDI) and thioredoxin in catalysing disulphide bond isomerization in a protein substrate were compared by using the standard assay, namely the re-activation of 'scrambled' RNAase. 2. The specific activity of PDI was 25-fold greater than that of thioredoxin. 3. The greater efficiency of PDI compared with thioredoxin is considered to be due more to the presence of multiple catalytic domains in PDI than to differences in their active-site sequences. 4. Data and procedures were defined for expressing enzyme activity in standard units, i.e. mumol of active RNAase generated/min.